A microelectronic package comprises a microelectronic die electrically interconnected with a carrier substrate, and one or more other elements, such as electrical interconnects, a die lid, a heat dissipation device, among others. An example of a microelectronic package is an integrated circuit microprocessor. Interconnecting contacts on the carrier substrate with those on a system substrate provides electrical communication between the microelectronic die and external components. An example of a system substrate is a printed circuit board (PCB) in the form of a motherboard for a desktop computer. The system substrate is commonly assembled into a system that is housed within a frame or chassis, for example, the enclosure of a desktop computer.
More powerful microelectronic die that operate at increasingly high temperatures require the use of larger and heavier heat dissipation devices on the microelectronic package. An example of a heat dissipation device includes a heat sink. The added weight of the heat dissipation device can cause the system substrate to flex and sag, especially under dynamic loading conditions experienced during handling of the system.
Surface mount technology (SMT) interconnects are especially susceptible to damage due to system substrate flexing and sag. One example of a SMT interconnect is the ball grid array (BGA) wherein contact pads on the surface of the SMT component are interconnected with contact pads on the surface of the system substrate using solder balls. Any loading that would tend to cause one of the surfaces to become non-flat will put stress on the solder interconnections. For example, on a commonly used desktop motherboard, the BGA to board interconnects of the chipset next to the CPU can be damaged due to board flexure caused by shock & vibration of the CPU heat sink.
Stiffening bars have been used to provide mechanical stiffening for system substrates to resist sag and flexing. These are essentially small I-beams that are wave soldered to the frontside of the system substrate through holes at regular intervals. The stiffening bars are placed at strategic locations and orientations on the system substrate to reduce deflection in selected directions. Component placement on the system substrate, however, must accommodate placement of the stiffening bars, putting constraints on the use of available standardized system substrate designs. In addition, the mounting holes required by the stiffening bars are relatively large. In the typical desktop system, stiffening bars are not feasible considering the component density requirements of the motherboard. They are used mainly for workstation and server systems that are less size constricted.
Stiffening plates and load transfer plates are also available. Stiffening plates are plastic or metal plates that attach to the backside of the system substrate by rivets. The rivets require relatively large holes that are difficult to accommodate in smaller systems. Load transfer plates are used to attach the backside of the system substrate to the chassis for additional support. Both stiffening plates and load transfer plates require access to the backside of the system substrate and a backside assembly step during manufacturing. Backside assembly is not a process that is normally used in common desktop motherboard manufacturing, for example, which leads to increased assembly cost and complexity.
Improved structural reinforcements are required to mitigate damage to the interconnects of SMT component due to system substrate flexing. The structural reinforcements must accommodate locations on the system substrate that have high trace densities, for example, the CPU/chipset core of a motherboard. Further, the structural reinforcements must not displace the layout of the components or traces. Further, the structural reinforcements must accommodate the heat dissipation devices used, for example, with core area components.